When used as part of a response to a preflight request, this indicates whether or not the actual request can be made using credentials. Note that simple GET requests are not preflighted, and so if a request is made for a resource with credentials, if this header is not returned with the resource, the response is ignored by the browser and not returned to web content.

The Access-Control-Allow-Credentials header works in conjunction with the XMLHttpRequest.withCredentials property or with the credentials option in the Request() constructor of the Fetch API. For a CORS request with credentials, in order for browsers to expose the response to frontend JavaScript code, both the server (using the Access-Control-Allow-Credentials header) and the client (by setting the credentials mode for the XHR, Fetch, or Ajax request) must indicate that they’re opting in to including credentials.